Charles Vess once told me the third book is a disappointment (I've not read that one) because of Peake's mental decline due to the illness that killed him. But the first two, no questions, are masterpieces.
This is not quite fantasy. Nothing strictly magical happens. But in the crumbling halls, secret passages, and forgotten rooms of Gormenghast, peculiar characters are endure surreal events (one is killed by owls; another becomes a feral child leaping from bough to bough in the surrounding forest; the Queen is trailed by a hundred white cats as she prowls the castle).
It has a satirical impulse, lampooning decrepit royal ritual, English education, and other mouldering institutions. But it's not exactly humorous .
The best term is probably poetically weird. It's the language that's special. So image-rich, with such sumptuous word-choice, that it's hard to read more than a few pages at a time. It's as filling as a creamy soup.
Imagine a Terry Gilliam movie written by Vladimir Nabokov and Tom Stoppard and you approach Peake's sensibilities.
(The BBC did film it as a miniseries, starring a young, scary Jonathan Rhys-Meyers. Not bad; could've used a hundred million more bucks, though.)
It's a one-of-a-kind literary experience.